Oxetane compounds give negative results in the Ames test even when their molecular weight is low and are highly stable and thermally polymerizable as described in the non-patent document 1 and they are suitable for use in areas such as coating materials, paints, adhesives, pressure-sensitive adhesives, and the like. In recent years, consideration of environmental hazards and product safety favors the use of oxetane compounds in aqueous solutions free of organic solvents in the aforementioned areas. However, no catalyst has been known that can polymerize oxetane compounds in the form of an aqueous solution. For example, a variety of oxetane derivatives and photopolymerizable compositions are described in the patent document 1; however, these compositions are neither water-soluble nor suited for thermal polymerization.    Patent document 1: JP3074086 B    Patent document 2: JPH9-208674 A    Patent document 3: JPH9-221625 A    Non-patent document 1: Journal of the Adhesion Society of Japan, 38 (12), 452 (2002)    Non-patent document 2: Kogyo Kagaku Zasshi, 63 (3), 541 (1960)    Non-patent document 3: The Chemistry of Cationic Polymerization, Symposium Publications Division, Pergamon Press, 1963; p 424
It is described in Kogyo Zairyo, 49 (6), 53 (2001) that quaternary onium salts such as tetraphenylphosphonium bromide are effective for the thermal polymerization of oxetane compounds. However, these quaternary onium salts are not soluble in water and cannot be used as an aqueous solution.
Resin compositions containing sulfonium salts as a catalyst are described as excellently capable of polymerizing at low temperature in the patent document 3. These sulfonium salts are commercially available in the tradename of San-aid SI series from Sanshin Chemical Industry Co., Ltd. However, as described in the product safety data sheets issued by the company, they decompose in contact with water and cannot be used as an aqueous solution.
Polymerizable oxetane compositions containing organosilanes and aluminum chelates as catalysts are described in the patent document 2. However, these catalysts are not soluble in water and react with water to form insoluble silica-alumina.
An example of the polymerization of an oxetane by a BF3-ether complex is described in the non-patent document 2; the Friedel-Crafts type cationic polymerization catalysts such as a BF3-ether complex and tin chloride are generally used for cationic polymerization, but they cannot be used in an aqueous medium as they are inhibited by water from participating in polymerization as described in the non-patent document 3. Thus, it has been difficult to polymerize oxetanes cationically in an aqueous solution in the presence of any of the conventional catalysts.